The present invention relates generally to a driver circuit for a self-scanning light-emitting element array, more particularly to a driver circuit without using current sources. The present invention further relates to a self-scanning light-emitting element array using such driver circuit.
A light-emitting element array in which a plurality of light-emitting elements are arrayed on the same substrate is utilized as a light source of a printer, in combination with a driver circuit. The inventors of the present invention have interested in a three-terminal light-emitting thyristor having a pnpn-structure as a component of the light-emitting element array, and have already filed several patent applications (for example, Japanese Patent Publication Nos. 1-238962, 2-14584, 2-92650, and 2-92651.) These publications have disclosed that a self-scanning function for light-emitting elements may be implemented, and further have disclosed that such self-scanning light-emitting element array has a simple and compact structure for a light source of a printer, and has smaller arranging pitch of light-emitting elements.
When such a self-scanning light-emitting element array is utilized for a printer, the light emission by the elements due to a transfer operation is not desirable. Then, a self-scanning light-emitting element array has proposed, which can be escaped from the effect of light emission due to the transfer operation (see Japanese Patent Publication No. 2-263668). According to this self-scanning light-emitting array, a shift portion for a transfer function and a light-emitting portion for a light-emitting function are separated and the light-emitting thyristors are used in both portions, and the light emission of the thyristors of the shift portion is shielded by covering the thyristors by metal or the like.
Alternatively, the light output during a transfer operation may be sufficiently suppressed by using a light-emitting thyristor having a characteristic such that the light output is small in a low current area as shown in an example of I-L characteristic of FIG. 1. In the figure, an abscissa designates a current (mA) and an ordinate a light output (xcexcW), and an arrow A shows a light emission during a transfer operation (hereinafter referred to as a transfer light-emission) and an arrow B a light emission during a write operation (hereinafter referred to as a write light-emission).
In such a case, it is unnecessary that the shift portion and light-emitting portion are separated, and a light-emitting element array serving as both the shift and light-emitting portions may be used for a light source of a printer.
In such self-scanning light-emitting element array, two kinds of currents, i.e. a current It required for the transfer operation (hereinafter referred to as a transfer current) and a current Iw required for the write operation (hereinafter referred to as a write current).
Referring to FIG. 2, there is shown an equivalent circuit diagram of a conventional self-scanning light-emitting element array to which the transfer current It and writing current Iw are applied. This self-scanning light-emitting element array is a two-phase (xcfx861 and xcfx862) driving type. In FIG. 2, T1, T2, T3, . . . designate light-emitting thyristors, D1, D2, D3, . . . coupling diodes, and R1, R2, R3, . . . gate load resistors. Each cathode of thyristors is connected a substrate electrode, each anode of odd-numbered thyristors T1, T3, . . . is connected to a clock pulse xcfx861 line 11, and each anode of even-numbered thyristors T2, T4, . . . is connected to a clock pulse xcfx862 line 12. Each gate of thyristors is connected to a power supply VGK line 14 via the load resistors R1, R2, R3, . . . respectively, and neighboring gate electrodes are connected to each other via the diode D1, D2, D3, . . . , respectively. Each of lines 11, 12 and 14 is connected to a driver circuit 62 via terminals 21, 22 and 24. The gate of light-emitting thyristor T1 is connected to a start pulse xcfx86s terminal 23.
In FIG. 2, reference numeral 10 designates the part integrated as a self-scanning light-emitting element array chip. The driver circuit 62 is added to the chip 10 externally.
The terminals 21, 22 and 23 of the chip 10 are connected to pulse voltage sources 51, 52 and 53 via current limiting resistors 41, 42 and 43, respectively, and the terminal 24 is connected to a voltage source 60. A pulse current source 31 is connected in parallel with the series circuit of the resistor 41 and the pulse voltage source 51, and a pulse current source 32 is connected in parallel with the series circuit of the resistor 42 and the pulse voltage source 52.
In the self-scanning light-emitting element array having the structure described above, the transfer current It is generated by the resistors 41, 42 and the pulse voltage sources 51, 52. In order to cause a desired thyristor to be in a write light-emission condition, the thyristor is turned on by a transfer operation and the required write current Iw is applied thereto by the pulse current sources 31 or 33.
FIG. 3 shows the wave shapes of the voltages generated by the pulse voltage sources and the currents generated by the current sources, and the conditions of the transfer light-emission and write light-emission of the thyristors. xe2x80x9cV (numeral)xe2x80x9d designates the voltage generated by the pulse voltage source referenced by the numeral in parentheses, xe2x80x9ci (numeral)xe2x80x9d the current generated by the pulse current source referenced by the numeral in parentheses, and xe2x80x9cL(Tn)xe2x80x9d the light output of the n-th thyristor Tn.
The light-emitting thyristors T1 is turned on, when the pulse voltage v (53) of the voltage source 53 for the start pulse is at L (Low) level, and the pulse voltage v(51) of the voltage source 51 for the clock pulse xcfx861 is at H (High) level. In the wave shape of the light output L(T1), xe2x80x9caxe2x80x9d designates the light output level of transfer light-emission, and xe2x80x9cbxe2x80x9d the light output level of write light-emission.
As described above, the on-state of the light-emitting thyristors is transferred by the repetition of the two-phase clock pulses xcfx861 and xcfx862 after the thyristor T1 is turned on. The turned-on thyristor emits the light, but the light output thereof is extremely small. The write currents i(31) or i(32) is applied to the thyristor from the pulse current sources 31 or 32 in order to cause the thyristor to be in a write light-emission condition.
According to the conventional self-scanning light-emitting element array described above, there is a problem such that the pulse current source has a complicated circuit and a characteristic which varies widely.
Also, the turned-on thyristor T5, for example, is in xe2x80x9cno-writing stagexe2x80x9d in FIG. 3, but a current is flowed via the thyristor T5 to hold it on-state, so that the thyristor emits the light slightly. The light-emission in xe2x80x9cno writingxe2x80x9d state causes a noise, so that its light output is desirable as small as possible. In order to decrease its light output, each resistance of the resistors 41 and 42 which determines the magnitude of the transfer current It is required to be enlarged. However, the problem is caused in that the increase of the resistance make the speed of the transfer operation slow.
An object of the present invention is to provide a driver circuit which can realize the same structure as a pulse current source in a simple circuit.
Another object of the present invention is to provide a driver circuit which has a function such that a light output may be decreased without lowering the speed of a transfer operation in xe2x80x9cno-writingxe2x80x9d state.
Further object of the present invention is to provide a self-scanning light-emitting element array which comprises such driver circuit.
A drive circuit according to the present invention is for a self-scanning light-emitting element array. The self-scanning light-emitting element array has such a structure that a plurality of three-terminal light-emitting elements are linearly arranged each having a control electrode for controlling threshold voltage or current, the control electrodes of neighboring light-emitting element are connected to each other via electrical means having unidirectional characteristic to voltage or current, two-phase clock pulses xcfx861 and xcfx862 are applied alternately to one of two terminals except the control electrode of each light-emitting element, one phase clock pulse of the two-phase clock pulses causes the threshold voltage or current of the light-emitting elements in the vicinity of a turned-on light-emitting element to vary via the electrical means, the other phase clock pulse of the two-phase clock pulses causes the light-emitting element neighbored to the turned-on light-emitting element to turn on, and the turned-on light-emitting element is caused to be in a write light-emission condition by supplying a write current thereto.
As each of the clock pulse xcfx861 and xcfx862 terminals of such self-scanning light-emitting element array has a constant voltage characteristic at an on-state of the light-emitting element, series circuits each consisting of a voltage source and a resistor are provided to the xcfx861 and xcfx862 terminals, respectively, so as to serve as current sources. As the voltage source, a buffer is used the input terminal thereof is connected to a power supply. The series circuit of the buffer and the resistor is provided between a pulse voltage source and the clock pulse terminal of a chip to input a pulse voltage outputted from the pulse voltage source to a gate terminal of the buffer.
On the other hand, a circuit for supplying the clock pulse also uses a series circuit of a buffer and a resistor to connect this series circuit to a pulse voltage source. A pulse voltage from the pulse voltage source is supplied to an input terminal of the buffer. In this case, a gate terminal of the buffer is generally grounded.
According to such driver circuit including the buffer and resistor, its structure is extremely simple compared with the conventional driver circuit, so that it may be implemented by a CMOS logic IC. The buffer furthermore may be integrated together with the resistor in the chip wherein the light-emitting elements are formed.
In order to resolve the problem such that the speed of the transfer operation is slowed when a light output is intended to be decreased in xe2x80x9cno-writingxe2x80x9d state, a resistance is made small during a transfer operation, and is made large after the transfer operation. Two sets of series circuit each consisting of a buffer and a resistor are provided and the ends of the resistors are connected together. The magnitude of a current flowed during a transfer operation and after the transfer operation may be changed by controlling the two sets of series circuits with the pulses supplied by pulse voltage sources, with the resistances of the two resistors being different, i.e. one is large and the other is small.
Also, such driver circuit may be implemented by CMOS logic IC, and may be integrated in a chip wherein the light-emitting elements are formed.